Long-standing controversy surrounds the question of whether living bird lineages emerged after non-avian dinosaur extinction at the Cretaceous/Tertiary (K/T) boundary or whether these lineages coexisted with other dinosaurs and passed through this mass extinction event. Inferences from biogeography and molecular sequence data (but see ref. 10) project major avian lineages deep into the Cretaceous period, implying their 'mass survival' at the K/T boundary. By contrast, it has been argued that the fossil record refutes this hypothesis, placing a 'big bang' of avian radiation only after the end of the Cretaceous. However, other fossil data--fragmentary bones referred to extant bird lineages--have been considered inconclusive. These data have never been subjected to phylogenetic analysis. Here we identify a rare, partial skeleton from the Maastrichtian of Antarctica as the first Cretaceous fossil definitively placed within the extant bird radiation. Several phylogenetic analyses supported by independent histological data indicate that a new species, Vegavis iaai, is a part of Anseriformes (waterfowl) and is most closely related to Anatidae, which includes true ducks. A minimum of five divergences within Aves before the K/T boundary are inferred from the placement of Vegavis; at least duck, chicken and ratite bird relatives were coextant with non-avian dinosaurs.

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The fossil record of the family Falconidae is poor and fragmentary. Extinct representatives from South America include the late early Miocene (Santacrucian) Thegornis musculosus and Thegornis debilis. Both species were originally described as Falconidae and afterwards moved to Accipitridae Circinae or Buteoninae. The analysis of a very well preserved and complete specimen of T. musculosus with similar stratigraphic and geographic provenances of the type material (lower levels of Santa Cruz Formation, coast of Patagonia, Argentina) corroborates the validity of the genus and its falconid affinities. The skull and postcranial morphology exhibit strong resemblances with the open-savannah inhabiting Herpetotheres and the forest-dwelling Micrastur (Herpetotherinae) but differ substantially from Falconinae (Falconini plus Caracarini). Detailed comparisons with a broad arrange of falconiform taxa in a cladistic framework, confirm its phylogenetic placement within the Herpetotherinae and sister to H. cachinnans. The ecotonal margins produced by the vanishing of humid forests that developed during changes in Patagonian plant communities throughout early Neogene times are hypothesized as a plausible scenario to understand the evolution of this basal clade of falcons.

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